1. Field of the Invention
The present invention relates to radio communication systems such as cordless telephone communication systems in which communication is performed on the time division multiple access (TDMA) basis, and more particularly, to a radio communication system in which data is transmitted by means of a TDMA frame using a plurality of slots in the TDMA frame by properly assigning the slots and the carrier frequencies.
2. Description of the Related Art
In a radio communication system such as a cordless telephone system, recent trend is to change its transmission system from an analog transmission system to a digital transmission system. In the digital radio transmission system, TDMA is generally employed because of its good transmission efficiency.
In a zone area where digital radio communication is performed between a plurality of cell stations and a multiplicity of portable stations, it is necessary to use a plurality of carrier frequencies for both the cell stations and the portable stations.
When a different carrier frequency is assigned for each slot in the TDMA frame at a cell station, a receiver of the cell station may not be able to follow the change in the carrier frequencies from one slot to another. In order to follow the carrier frequency change, the cell station is provided with two or more receivers which are sequentially selected to receive respective slots.
For the portable stations, usually it is sufficient to be provided with a single receiver since one of the slots is usually used in a TDMA frame.
However, when a portable station uses a plurality of slots in a TDMA frame, it must be provided with two or more receivers. Thus, the portable station cannot be light-weighted and small-sized, and has a complicated circuit arrangement.
Further, since there are a number of the portable stations in the zone area, the radio communication system becomes very costly.